This invention relates to WC (tungsten carbide)-base sintered alloys and a method for their production.
WC-base sintered alloys are used as various industrial materials, for example, cutting tools, metal forming tools (such as wire drawing dies or blanking dies), mining tools and wear-resistant tools because of their high mechanical strength and hardness and superior wear resistance. WC-base sintered alloys for cutting tools are used in a very wide range since it has higher toughness than other materials such as TiC(titanium carbide)-base sintered alloys, TiN (titanium nitride)-base sintered alloys and alumina-base sintered ceramics, and undergo little damage during a machining operation. However, the WC-base sintered alloys have inferior chemical stability as demonstrated by the fact that they are more susceptible to reaction with a workpiece and to oxidation than the other sintered alloys. Thus, they have the defect that when used as cutting tools, they tend to undergo crater wear and flank wear during high-speed machining. With the higher speeds of machining employed in recent years, the range of the use of the WC-base sintered alloys has gradually been narrowed.
Various methods have been used to remove the defects of the WC-base sintered alloys, but the addition of TiC is the most widely practised method.
A WC-TiC-Co sintered alloy has improved chemical stability, but suffers from markedly reduced mechanical strength and thermal conductivity. This is probably because while a WC-Co sintered alloy is composed of a WC phase and a bonding metal phase the WC-TiC-Co sintered alloy contains as a third phase a covering layer of a complex carbide of titanium and tungsten [(WTi)C] surrounding the WC layer, and this covering layer is brittle and during sintering, tends to be subject to grain growth. Thus, addition of much TiC in an attempt to improve resistances to crater wear and wear at the time of high-speed machining results in a marked reduction in strength, and tends to cause damage.
Accordingly, the tool manufacturers provide alloys with varying amounts of TiC. It has also been attempted to make WC-base sintered alloys containing TaC (tantalum carbide), NbC (niobium carbide), VC (vanadium carbide) and Mo.sub.2 C (molybdenum carbide). But all of these alloys have the defect that the strength of the WC-Co alloy is reduced.